This invention relates to a pneumatic radial tire which improves shoulder wear at shoulder portions, and more particularly to a pneumatic radial tire for a heavy load.
In pneumatic radial tires, particularly in pneumatic radial tires for a heavy loads, improvement in wear resistance of a tread has been the most important problem. In conjunction with this wear resistance, a rib-based pattern has higher resistance than a block pattern, and the wear resistance can be improved much more by increasing a tread expansion width. Although the increase of the tread expansion width is effective for improving the wear resistance, there remains the problem that because a ground contacting pressure increases locally and remarkably at an end portion of a shoulder, shoulder end wear of the shoulder portion increases.
To reduce such shoulder end wear of the shoulder portion, a method which disposes a thin groove 26 at the end portion of a shoulder rib 1s in a tire circumferential direction as shown in FIG. 3 and thus reduces a peak ground contacting pressure at the shoulder end portion has been proposed in the past. However, according to this method, the thin groove 26 is likely to catch gravel, and the gravel so caught generates in turn cracks on the groove bottom. Furthermore, fall-off of outside thin ribs 25, that is, rib tear, is likely to occur.
To prevent this catch of gravel, a proposal has been made which continuously disposes the thin groove 26 in the tire circumferential direction in such a manner as to open in the transverse side surface of the tread portion as shown in FIG. 4. This counter-measure involves the problem that when a load to the tire is relatively small, the inner wall of the thin groove 26 on the outer diameter side so changes as to approach the groove wall on the inner diameter side as shown in FIG. 6A and provides the effect of reducing the ground contacting pressure of the shoulder end portion, but when a heavy load is applied to such an extent that the inner wall of the thin groove 26 on the outer diameter side comes into complete adhesion with the inner wall on the inner diameter side as shown in FIG. 6B, the ground contacting pressure drastically rises, so that the effect of reducing the ground contacting pressure becomes smaller than that of the counter-measure shown in FIG. 3. Moreover, because the inner wall on the outer diameter side and the inner well on the inner diameter side of the thin groove 26 repeat opening and closure with the revolution of the tire, the stress concentrates on the groove bottom, and cracks are more likely to occur due to this stress concentration.
To prevent the cracks occurring at the groove bottom of the thin groove, Japanese patent application Kokai publication No. 1-233103 proposes a method which disposes an extension portion which is curved towards the inside in the radial direction at the groove bottom of the thin groove 26 of FIG. 4, as shown in FIG. 5. This method provides the effects of reducing the cracks by dispersing the stress concentration by the curved extension portion of the thin groove 26, but cannot solve the problem of FIG. 4 in that the ground contacting pressure rises when a heavy load above a predetermined level is applied. Accordingly, it cannot sufficiently improve the uneven wear resistance. Furthermore, the thin groove 26 having the curved extension portion makes it difficult to fabricate the mold and to release the tire after molding from the molds, so that the tires are likely to be damaged.